1. Field of the Invention
The present invention is directed to the qualitative and quantitative analysis of elements in a sample when present in medium to ultra-low concentrations, by measurement of the wavelength and intensity of their fluorescence when the sample is introduced as a vapor in a gas stream containing an excess of energetic metastable species of nitrogen or an inert gas.
2. Description of the Prior Art
There is an abundance of literature and many varieties of apparatus in the field of the analysis of the elements and their compounds. Two of the most widely used techniques are atomic and molecular emission spectrometry and atomic absorption spectrometry. These techniques are generally recognized to have lower limits of sensitivity and accuracy to concentrations of 10.sup.8 atoms per cubic centimeter (cm.sup.3). Particularly with the advent of lasers, atomic fluorescence spectrometry has been improved to provide sensitivities to less than several hundred atoms per cm.sup.3 with some experimenters reporting the detection of a single atom. Under laser excitation at a wavelength within the absorption spectrum of the atomic species under study the measurement of the resulting atomic fluorescence has emerged as one of the most sensitive techniques in spectroscopy. This sensitivity is provided by the atomic species under laser illumination participating repeatedly in the almost simultaneous excitation-emission process whereby a plurality of signals is produced by each atom. Unfortunately there are wavelength limitations on methods employing laser excitation. The principal limitation is that any single laser cannot normally be used to excite several different atomic species simultaneously, and certain species cannot at all be analyzed because of the unavailability of a laser with the required absorption-matching wavelength. Other disadvantages of using laser excitation are the expense of sophisticated lasers and, in some cases, the capability of certain lasers to operate only in a pulsed mode, thus reducing the sensitivity of the method.
The present invention employs an active metastable species of a gas, such as nitrogen or the inert gases, to excite atomic species, particularly, but not limited to, the metallic elements, to fluorescence. The active metastable is also capable of dissociating compounds and subsequently exciting the resulting constituent atoms. This phenomenon is generally reported in the literature. One of the most pertinent discussions of the fluorescent emissions of certain metals by collision with active nitrogen is offered by L. F. Phillips, Canadian Journal of Chemistry, Volume 41 (1963) 2060-2066. Phillips, as well as other researchers, recognize generally that the fluorescent emission of the metallic atoms in the presence of active nitrogen varies in intensity in some ratio to the concentration of the metal. In all known reported works, it is particularly noteworthy, however, that only high concentrations of additives were employed in the experiments and the experimenters were not concerned with precise quantitative analysis of the fluorescing atoms or their compounds. Moreover, it is apparent from such as the Phillips report that the phenomenon of fluorescence of atoms by active nitrogen excitation was not accurately or completely understood, and for this reason prior researchers failed to recognize its application to the quantitative and qualitative analysis, especially of ultra-low concentrations of elements.
Phillips' reported work was limited primarily to the excitation of metal halides by active nitrogen. He explains that in a one-step reaction the metal halide is dissociated by active nitrogen, producing the halogen and the metal in an excited state, with the metal fluorescing at its characteristic wavelength. From his experiments and observations Phillips concludes that thallium and lead atoms are not excited to any significant degree by active nitrogen, but rather that the metals are efficiently excited to fluoresce only upon dissociation from a molecular state. Thus, the theory of the simultaneous activation-dissociation-fluorescence sequence as deduced by Phillips concluded that each molecule of the metallic compounds (e.g., thallium halide) fluoresces only once. Thereafter, the dissociated metal atoms were observed by Phillips not to be reactivated to fluorescence. In this connection it must be realized that Phillips' analysis was based on his observations of the experiment as actually performed.
The prior art has overlooked the use of active-nitrogen-induced fluorescense of atoms to detect extremely low concentrations thereof. In fact, some workers in the prior art, such as Phillips (discussed above) have concluded that the process is not an effective analytic tool for high concentrations of atomic species. Perhaps because of this conclusion, no prior attempt was directed to its application to the analysis of very low concentrations in the vapor phase. We surmise the prior art may have been directed away from our invention because previous apparatus for this process may have been constructed in such a manner as to disperse the material ineffectively into the gas flow containing the active nitrogen or was otherwise not properly tailored for the purpose of our invention. Also, since at high concentrations of sample material the active species can be quenched, thus producing only a very low rate of excitation, it might have been erroneously concluded that such rate would not produce a meaningful intensity of fluorescense at low concentrations.